They do get donations for that and a lot of people including myself have them on their auto vote list which helps them to pay for the server. A while back they explored the option to charge some kind of fee but it did not come to that so for now it is all free to use and it has been working very consistently for me. There are a couple more options out there for auto-voting. https://steemdunk.xyz/ (paid) & https://steemautomated.eu/ (free) along with Steemvoter.